A digital image processing method has been proposed wherein a sample of tone values from the information portion of the image is normalized to produce a tone reproduction function. The tone reproduction function is then applied to the digital image to produce a processed digital image. This procedure was first proposed in the article entitled "A New Approach to Programming in Photomechanical Reproduction" by Yu. Ovchinnikov et al., the 12th IARIGAI Conference Proceedings, Versailles, France, Editor: W. Banks IPC Science and Technology Press, Guildford, England 1974, pages 160-163. Improvements to this method are disclosed in copending U.S. patent application Ser. Nos. 730,630 and 730,629, wherein the sample of tone values is selected from an image-dependent "floating" contrast interval selected as a function of the statistical characteristics of the tone value distribution in the contrast interval; and the overall contrast of the processed image is adjusted by multiplying processed tone values by a constant function of the standard deviation of tone values in the samples used to form the tone reproduction function. An extension of this method to processing digital color images to adjust tone-scale and color balance by histogram modification is disclosed in copending U.S. patent application Ser. No. 730,627.
This method produces an aesthetically pleasing processed image in most instances. However, when the tone values in the image are bimodally distributed, such as the case for back-lit photographic scenes, and some close-up flash photographic scenes, further improvement in the appearance of the processed image is desirable.
It is the object of the present invention to provide an improved digital image processing method of the type described above, wherein the appearance of processed images having a bimodal tone value distribution is further improved.